This invention relates generally to the field of electrical cable connecting apparata. More specifically, it relates to a center conductor seizure apparatus of particular use in a community antenna television transmission system, often referred to as a CATV system.
In communication systems in general, coaxial cables are almost exclusively used for handling the radio and ultra-high frequency wave energies necessarily encountered. Such cables typically include elongate center and outer conductors having a common axis and separated by a dielectric medium which can be air, or a synthetic such as polyethylene foam. The center conductor carries the high frequency signal and is of single or multi-strand wire design, being made of copper, copper-plated aluminum, or some other plated conductive metal. The outer conductor is of tubular design, is aluminum or some other alloyed conductive metal, and serves to shield the center conductor from external forces that may interfere with signal transference, as well as to provide an electrical ground for the system. The dielectric spacer material holds the two conductors in coaxial alignment and also helps to insulate the center conductor from corrosive elements.
These coaxial cables have proved highly effective in handling the frequencies experienced with CATV systems. However, problems have arisen in the design and construction of suitable connecting means for intermediate and terminal connecting with such cable lines.
As the name implies, a CATV system contemplates the use of a central antenna situated as to best receive a number of differing television signals emanating from various sources. The antenna is usually remote from the ultimate consumers, and the signals must therefore be transmitted to the television sets of the individual subscribers by means of coaxial cables connected through a series of intermediate devices. Such devices include signal amplifiers, line splitters, direction couplers, and other active and passive devices which are interposed at various points in a given cable to direct signals along various secondary transmission paths. It is the mechanical and electrical connection at these various points that has caused continual problems for the CATV systems.
Another of these intermediate devices is a subscriber tap, also known as a tap connector box, a multi-tap, a cable substation, a directional device and the like. Regardless of its name, this tap forms the interface between the cable system and the ultimate customer. In particular, there exist secondary, or "feeder", cables which branch from a primary, or "trunk," line to service smaller residential areas by transmitting signals to these subscriber taps. Each tap in turn connects into the signal path and carries the signal directly to the television sets of a small number of individual subscribers.
These subscriber taps generally include a box-like, conductive housing having a removable base and a pair of coaxial cable access ports orthogonally positioned at each of two corners on the housing structure to signify the signal input and output sides of the housing. These alternative input and output ports attest to the two common methods of CATV installation. First is an aerial installation in which the housing is typically suspended from a metal strand with cable access being through opposite sides of the housing to minimize bending of the coaxial cable. Second is an above- or below-ground installation in which a pedestal mounting is used with cable access generally on the same side of the housing for convenience. An "L" installation is also possible with the pairs of orthogonal ports; but regardless of the particular installation used, the unused ports are generally plugged or capped to maintain the integrity of the inner housing chamber.
Electrical circuitry is then provided in the housing connecting to the seized center conductors and transmitting the signal to a number of individual subscribers through subscriber connectors and cables attached to the base cover of the subscriber tap.
Two problems of mechanical and electrical connection are encountered with these subscriber taps.
First, some form of cable connectors means must be used to mechanically and electrically connect the outer cable conductor with the housing at its point of entry through an access port. Such connector means are known in the art. For example, Forney, Jr., U.S. Pat. No. 3,778,535, issued Dec. 11, 1973, discloses a coaxial connector designed to avoid collapsing the outer cable conductor while not altering the characteristic impedance of the coaxial cable, determined as a function of its inner dimensions, thereby avoiding any reflective loss in the frequency signal being transmitted. Hayward et al., U.S. Pat. No. 3,847,463, issued Nov. 12, 1974, discloses a related connector apparatus designed to withstand the various rotational and vibrational forces experienced by the cable through wind and otherwise without the eventual fatigue and rupture experienced with other connectors.
The second area of contact involves the mechanical and electrical seizure of the center cable conductor within the housing chamber. This seizure is typically accomplished by the use of a screw means clamping the center conductor is a connector device after its insertion through one of the access ports. Problems are also common with these connectors.
For example, Lankford et al., U.S. Pat. No. 3,675,181, issued July 4, 1972, discloses a connector claiming an improved seal means and supposed to avoid the normal conductor loosening encountered with prior art devices as a consequence of a metallurgical phenomenon known as "cold flow." Other related connecting devices have concentrated on the problems of shearing and other damage to the center conductor caused by cross screw connectors, such as Cauldwell, U.S. Pat. No. 3,989,333, issued Nov. 2, 1976, and Nordberg et al., U.S. Pat. No. 4,025,150, issued May 24, 1977.
Still two further connector apparatus are shown in Vachhani, U.S. Pat. No. 3,530,425, issued Sept. 22, 1977, and Devendorf, U.S. Pat. No. 3,951,490, issued Apr. 20, 1976. Vachhani securely holds the center conductors of connecting cables between the serrated contacts of its pressure terminals. Devendorf, on the other hand, discloses a substation seizure apparatus secured to the housing by a plurality of standing projections claimed to distribute the forces exerted by the center conductor throughout the assembly frame.
Still further problems exist unresolved by the subscriber taps known to date.
For example, the majority of seizure mechanisms now marketed continue to permit the shear of center conductors during installation. To provide for alternatively using the holes in the seizure device for both the seizure screw and the center conductor, these manufacturers drill their devices completely through from both the pedestal and aerial directions, tapping each hole to alternately accommodate either the screw or the center conductor. Therefore, when the seizure screw is tightened down against the center conductor, directly behind the center conductor is a hole causing the clamp to act like a punch and die set with respect to the conductor wire. If tightened too far, the screw will shear the center conductor by pushing it into the hole. Even if not severed, the conductor is often sufficiently damaged by pressing against either the back opening or the cutting threads of the tapped hole to fail under temperature cycles as the various expansion and contraction forces are exerted by temperature changes in the cable system.
Another problem concerns material selection. For instance, tin-plated brass connectors are known for long-term durability and are used in all terminators, drop connectors and expensive addressable taps used in the CATV industry. However, some manufacturers use aluminum spigots which under atmospheric conditions form a highly corrosive galvanic cell interface with a tin-plated brass contact thereby becoming welded, stuck, or corroded together.
A still further problem involves the basic understanding of an ideal subscriber tap assembly. It must provide a lasting mechanical and electrical connection with the center conductor. It must provide an impedance match with the coaxial line to minimize signal loss or ghosting due to signal reflections. It must be impervious to weather conditions and provide a connection as strong as the cable itself. Finally, it must be capable of installation and change quickly and easily in the field under all anticipated conditions.
This last concern for the speed and ease of tap installation and change is of great importance. The majority of taps on the market provide for tightening the seizure screw through the unused orthogonal access port in the housing corner. To change from aerial to pedestal mounting, or vice versa, requires the installer to completely remove the seizure screw, insert the center conductor through the new access port, and then reinsert the screw through the alternate port in order to secure the proper connection. This operation is burdensome and time consuming to the installer. It also promotes conductor shear because the conductor is pressed against the serrated threads on the inside wall of either seizure bore while further being pressed into the back opening.
The only subscriber taps on the market not using this disassembly procedure provide for vertical seizure of the center conductor by placing two separate access ports for the seizure screws in the housing wall opposite the base or the base itself. Center conductor seizure through alternate ports then does not require complete removal and reinsertion of the seizure screw. It does, however, require manipulation of another pair of access ports and caps. In addition, one such manufacturer bores completely through its clamp in the vertical direction thereby providing the same shearing problem during use.
This requirement of speed and ease in subscriber tap installation or change also concerns the accessibility of the inner housing chamber for repair of replacement of the seizure mechanism or the printed circuitry used to connect to the cable center conductor. In this regard, complex disassembly procedures must be avoided so that repair or replacement of internal parts can be accomplished quickly and easily in the field.